1. Field of the Invention
The invention relates to electro-optic devices and in particular to waveguide array devices and digital optical devices.
2. Description of Related Art
Optical waveguides, either active or passive have been fabricated from semiconductor, inorganic or organic materials. Optical waveguide switches have been reported in telecommunication and sensor applications.
Waveguides can be fabricated from a number of different materials, most of which may be categorized as crystalline or amorphous. Such materials exhibit an electro-optic effect that is broadly defined to include changes in the index of refraction, as well as an electro-absorptive effect, which is a change in absorption, and both effects are caused by the application of an electric field to a material through which light is propagating. A semiconductor waveguide can be fabricated by growing successively crystalline layers, for example, successive layers of GaAlAs, GaAs, and GaAlAs. Other examples of materials that are useful in fabricating waveguides are lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), zinc oxide (ZnO), and glassy polymers, doped with non-linear optical moieties.
This ability to alter the index of refraction with an electric field is useful for fabricating optical integrated circuit switches, modulators, and directional couplers. Single mode waveguides on the order of several wavelengths in cross-section are being constructed in electro-optic media making use of the technology used to make microelectronic circuits. Laser light is coupled into these guides and directed through different types of switching elements that can switch as fast as or greater than electrons can be directed through electronic chips. These circuits are generally referred to as photonic chips and have considerable potential for use in optical recording, telecommunication, computing and imaging graphics.
European Patent Application 338 864 A2 (Miyauchi et al.) discloses an integrated magneto-optical playback head using a discrete light source, a waveguide to direct the light onto the optical disc and a second waveguide to guide the reflected light for differential detection by photodetectors.
An integrated-optic disc pickup device (IODPD) has been proposed by Suhara et al., "An integrated optic disc pickup device," 5th IOOC-11th ECOC 1985, Venezia, Italy, October 1985). The IODPD device was constructed by integrating a focussing grating coupler, a grating beam splitter and photodiodes in a thin film waveguide on a single substrate. The device is a single channel device and is suitable for a non-erasable optical disc media. Although the inertia of the system is reduced in this integrated pickup device, focussing grating coupler is wavelength sensitive.
European Patent Application 0 174 008 (Inoue et al.) describes an integrated optical head using Fresnel lenses, focussing grating couplers and displaced photodiodes for tracking, error and signal detection.
U.S. Pat. No. 4,801,184 (Revelli) describes an optical head for reading and writing on non-erasable optical disc where the laser light is guided in a slab waveguide and is focussed onto the optical disc by means of an electrooptic focussing grating coupler fabricated on the same substrate. The reflected light is differentially read by means of photodetectors positioned on the opposite side of the substrate for focussing and tracking control. This invention is useful for bit-at-a time recording and is not suitable for magneto-optic media.
U.S. Pat. No. 4,796,226 (Valetta) describes a novel integrated optic reading head for reading information from an optical disc. The head reads information in a serial fashion using an interference technique. Unlike conventional optical heads, the optical head described in Valetta requires only a small spacing between the disc and the optical head.
Several parallel addressing schemes using laser arrays have been reported in the literature, see for example, K. Nishimura, and S. Murata, Paper No. WA2-1, Optical Data Storage Topical Meeting, Jan. 17-19, 1989, SPIE and W. P. Altman et al., 31-1 RCA Engineer (January/February 1986). In these cases 5-9 laser diode arrays are used to address 5-8 bits on an optical disc. Temperature rise of the laser arrays, mode hopping, wavelength shift and yield are some of the associated problems of the laser array scheme.
U.S. Pat. No. 4,074,085 (Russell) describes an optical recording playback apparatus for simultaneously scanning a plurality of data tracks on an optical record. The device uses separate laser beams, bulk optics and galvos for scanning. The system is not an integrated optic device scheme.
U.S. Pat. No. 4,298,974 (Tsunoda et al.) describes an optical head for a videodisc player/recorder comprising a semiconductor laser array as light source for reading and writing multiple tracks and an optical system that guides laser beams from the array to a predetermined recording medium.
U.S. Pat. No. 4,070,092 (Burns) describes a T switch as an active optical-waveguide branch that includes a single main waveguide branched into two element arms or paths such that optical radiation passing through the main waveguide will be normally divided equally into each branched arm. Electrodes, formed between the branching arms can selectively be made positive or negative with respect to a center electrode, thus permitting the selection of one branch of the Y switch versus the other, depending on the polarity of the potential applied.
U.S. Pat. No. 4,775,207 (Silberberg) describes a digital optical X switch having a switching on/off ratio greater than 1000:1. The electro-optic switch is implemented on a substrate which includes first and second waveguides of unequal width and a central region in which light from the two input waveguides converge. Electrodes for generating an electric field are disposed adjacent to the central region and for switching beams of light from the first to second output waveguide in a step-like manner.